Surface feature mapping using high resolution C-scan ultrasonography

ABSTRACT

An ultrasonic system and method for imaging a surface wherein a C-mode ultrasonic scan is performed over a fixed area of the surface and range gating is applied to that area to a given depth below the surface. For use of the system and method in fingerprint imaging, a live finger is placed upon a sensitive surface, the portion of the finger on the surface is scanned using the ultrasonic energy, and ultrasonic energy returned from the finger portion is received to capture an electronic image of the pattern of ridges and valleys of the fingerprint. The ultrasonic imaging system comprises a probe for providing a directed output ultrasonic beam to scan the surface and to receive ultrasonic echos from the surface, a pulser-receiver to cause the probe to provide the output beam and to provide signals in response to the returned ultrasonic echos, a signal processing circuit for detecting and processing return echo signals from the pulser-receiver and a computer for storing and displaying information contained in signals from the processing circuit and for controlling operation of the processing circuit. The probe scans the surface along one direction, and then along another direction, the two directions preferably being orthogonal.

This is a continuation application Ser. No. 08/003,800 filed on Jan. 13,1993 now abandoned, which is a division of Ser. No. 07/610,429 filedNov. 7, 1990 now U.S. Pat. No. 5,224,174 issued Jun. 29, 1993.

BACKGROUND OF THE INVENTION

This invention relates to the art of surface scanning and imaging, andmore particularly to a new and improved ultrasonic method and apparatusfor surface scanning and imaging.

One area of use of the present invention is in fingerprint scanning andimaging, although the principles of the present invention can bevariously applied to imaging surface topology using ultrasound. Inoptical techniques for fingerprint scanning, reflections from small airpockets under the fingerprint ridges reduce the image quality therebyrequiring image processing techniques which are quite complex and costlyto implement and which themselves can cause unwanted artifacts orpossibly remove valid components of the image. Another problem withoptical techniques is that once the ridge structure of a finger is wornsmooth enough optical systems no longer are able to acquire good qualityimages.

It would, therefore, be highly desirable to provide a system and methodfor imaging surface topology which provides high quality images therebyreducing the complexity and cost of subseqent image processing andwhich, in the case of personal identification, has the capability ofimaging structures which lie beneath the surface of the skin which canbe used for identification.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of this invention to provide a newand improved system and method for imaging surface topology.

It is a further object of this invention to provide such a system andmethod which provides high quality images so as to reduce the complexityand cost of subsequent image processing.

It is a more particular object of this invention to provide such asystem and method for use in fingerprint scanning and imaging.

It is a further object of this invention to provide such a system andmethod for use in personal identification which has the capability ofsubdermal imaging.

It is a more particular object of this invention to provide such asystem and method which is efficient and effective in operation andwhich is relatively simple in structure.

The present invention provides an ultrasonic system and method forimaging a surface wherein a C-mode ultrasonic scan is performed over afixed area of the surface and range gating is applied to that area at aselected location from the surface to a given depth below the surface.For use of the system and method in fingerprint imaging, a live fingeris placed upon a scannable surface, the portion of the finger on thesurface is scanned using the ultrasonic energy, and ultrasonic energyreturned from the finger portion is received to capture an electronicimage of the pattern of ridges and valleys of the fingerprint.

The ultrasonic imaging system comprises a probe for providing a directedoutput ultrasonic beam to scan the surface and to receive ultrasonicechoes from the surface, a pulser-receiver to cause the probe to providethe output beam and to provide signals in response to the returnedultrasonic echoes, signal processing means for detecting and processingreturn echo signals from the pulser-receiver and a computer for storingand displaying information contained in signals from the processingmeans and for controlling operation of the processing means. The probeincludes first means for scanning the surface along one direction,second means for scanning the surface along another direction, the twodirections preferably being orthogonal. In accordance with anotheraspect of the present invention, the fingerprint image is analyzed inthe spatial frequency domain.

The foregoing and additional advantages and characterizing features ofthe present invention will become clearly apparent upon a reading of theensuing detailed description together with the included drawing wherein:

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a diagrammatic view illustrating an aspect of opticalfingerprint scanning;

FIG. 2 is a diagrammatic view illustrating aspects of depth ofpenetration in optical scanning;

FIG. 3 is a diagrammatic view illustrating reflection and transmissionof ultrasound at an interface;

FIG. 4 is a graph in the form of the resonant curve of an ultrasonictransducer;

FIG. 5 is a diagrammatic view illustrating behavior of incident and echoultrasonic pulses at two interfaces;

FIG. 6 is a diagrammatic view illustrating lateral resolution of returnultrasonic echos;

FIG. 7 is a diagrammatic view illustrating depth of focus for twodifferent ultrasonic transducers;

FIG. 8 is a diagrammatic view illustrating Snell's Law for ultrasoniclenses;

FIG. 9 is a diagrammatic view illustrating a converging ultrasonic lens;

FIG. 10 is a longitudinal sectional view of an ultrasonic transducerused in the system of the present invention;

FIG. 11 is a block diagram of the ultrasonic imaging system according tothe present invention;

FIG. 12 is a graph including waveforms illustrating operation of thesystem of FIG. 11;

FIG. 13 is a longitudinal sectional view of the probe of the system ofFIG. 11;

FIG. 14 is a top plan view of the probe of FIG. 13;

FIG. 15 is a diagrammatic view illustrating a sector scan swept by theacoustic mirror in the probe of FIGS. 13 and

FIG. 16 is a diagrammatic view illustrating a relationship between theacoustic mirro and lens in the probe of FIGS. 13 and 14;

FIG. 17 is a diagrammatic view of a ray trace of the ultrasonic beamfrom the probe of FIGS. 13 and 14 as the beam strikes a scatterreflector;

FIG. 18 is a diagrammatic view of a ray trace of the ultrasonic beamfrom the probe of FIGS. 13 and 14 as the beam strikes a specularreflector;

FIG. 19 is a diagrammatic view illustrating ultrasonic return pulsesseen by the transducer in the probe of FIGS. 13 and when echoing aspecular reflector;

FIG. 20 is a diagrammatic view illustrating ultrasonic return pulsesseen by the transducer in the probe of FIGS. 13 and when echoing ascatter reflector;

FIG. 21 is a schematic diagram of a circuit for implementing the systemof FIG. 11;

FIG. 22 is a program flow chart illustrating the software forcontrolling operation of the computer in the system of FIG. 11;

FIG. 23 is a graph including waveforms providing a timing diagramillustrating operation of the program of FIG. 22;

FIG. 24 is a block diagram illustrating a character recognition systembased on spatial filtering;

FIG. 25 is a schematic diagram of an illustrative optical spatialfiltering system;

FIG. 26 is an enlarged image of a portion of a finger print illustratinga bifurcation and a ridge ending;

FIG. 27 is an enlarged grey scal finterprint image;

FIG. 28 is a spatial frequency representation of the fingerprint imageof FIG. 27;

FIG. 29 is an enlarged image of a fingerprint bifurcation and thepsatial frequency representation thereof;

FIG. 30 is an enlarged image of a fingerprint ridge edning and thespatial frequency representation thereof;

FIG. 31 is an enlarged image of a fingerprint parallel ridge structureand the spatial frequency representation thereof;

FIGS. 32a-f are enlarged images of spatial freuqncy representations ofdetected minutia from a spatial frequency analysis of the fingerprint ofFIG. 27; and

FIG. 33 is a map of the detected minutia of FIG. 32.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

A more complete understanding of the ultrasonic method and apparatus forsurface scanning according to the present invention perhaps can beobtained best from a brief consideration of optical systems. The almostunlimited number of optical imaging systems presently in existence use amultitude of different approaches to scanning that are targeted at manydifferent applications. However, for purposes of illustration, onlythose systems that rely on Frustrated Total Internal Reflection or FTIRas the basis for obtaining an image will be considered. The theory ofFTIR is that light incident on an interface going from a higher index ofrefraction to a lower one will be totally reflected if the incidentangle is large enough. If on the other hand the incident of refractionbetween the two surfaces is closely matched, a significant amount ofabsorption and scattering takes place. Several systems use this conceptas the basis for generating an image. One of the more classical examplesand the one which will be emphasized herein is that of fingerprintscanning. Referring to FIG. 1, a finger 1 is placed upon an opticalinterface such as a prism 2 and a light source (usually a laser) scansthe finger to obtain an image, relying on the ridges of the finger tocompletely contact the surface of the prism thus causing the lightsource to be scattered. However, often small air pockets form under theridges thereby reflecting the light as opposed to scattering it. Thiscreates a very poor quality image that must be improved by imageprocessing techniques. Often, these image processing techniques oralgorithms cause unwanted artifacts or possibly remove valid componentsof the image. Always, these techniques are quite complex and costly toimplement. However, prior to understanding the problems associated withscanning the fingerprint, general fingerprint pattern recognition theorymust first be understood.

A variety of fingerprint processing algorithms have been developed andexperimented with over the years, each with a varying degree of success.The basic idea behind all of these algorithms is to identify and locateunique points of the fingerprint referred to as minutia. The twopredominant types of minutia are ridge endings and bifurcations. A ridgeending is formed when a ridge of a fingerprint no longer continues alongits path, it simply stops or ends. A bifurcation on the other hand isformed when a ridge of a fingerprint splits (bifurcates) into two ridgesor, conversely, when two ridges merge into one ridge. Fingerprintidentification algorithms are concerned with identifying every minutiaof the fingerprint (both ridge endings and bifurcations) and associatingwith each minutia found, three positional identifiers (x, y, and theta).These three parameters locate the minutia in an arbitrary (but fixed)cartesian coordinate system where x and y map the position of theminutia and theta defines its angle of orientation with respect to oneof the axes. A match between two fingerprints is made when the x, y, andtheta of one fingerprint match (or nearly match) the x, y, and theta ofanother print.

Essentially, there are two basic methodologies which have been used toprovide an image of the fingerprint to be processed. The first techniqueis to generate an inked impression of the fingerprint. This is done byapplying ink to the finger to be printed and rolling the finger onto apiece of paper or cardboard. The inked image is then placed under anoptical scanner where it is scanned, digitized and placed into thememory of the computer responsible for processing the fingerprint. Thereare, however, a number of problems and deficiencies with this approachespecially when viewed in the realm of security systems. The first andforemost deficiency with this approach is the need to ink anindividual's finger or hand. Certainly in some applications such as lawenforcement, the inconviencing of the individual being printed is not aprimary concern. However, if the application is general security such asthat which is required for access control, the inconviencing of theindividual is of prime importance and generally rules out the use of anytype of inking procedure.

The second concern with this approach falls into the category ofequipment and material use. Over several years it has been demonstratedthat standardizing the type of ink to be used, along with the materialit is to be printed on, is not as trivial of a problem as it appears.Furthermore, different inks and different papers all affect the overallfinal quality of the image as it is scanned into the computer.

The third (and certainly not last) problem with this procedure is thetraining of the individual responsible for obtaining the print. Factorssuch as too little ink, too much ink, improper pressure on the fingerwhen rolling, etc., greatly affect the overall outcome of the image. Inaddition, if the individual to be printed is resistant to being printedin any way, the potential for obtaining a good quality print is farless.

The second methodology for obtaining an image of a fingerprint is toscan the finger directly. This approach is referred to as "live scan".The idea behind the devices used to scan the finger is based on theconcept of Frustrated Total Internal Reflection or FTIR. Live scanreaders that employ this technique rely on the fact that the interfaceestablished between the finger and the optical surface (usually a prismbut perhaps a lens) can generate both reflection and scattering basedupon the relative indices of refraction of the fingerprint valleys andridges versus the glass prism. That is, the prism has a high index ofrefraction. When interfaced to a valley of a fingerprint (i.e. air)which has a low index of refraction, the light is reflected back to aphotosensor for data conversion and storage. When the prism isinterfaced to a much higher index of refraction such as skin (i.e., theridge of the fingerprint), the light is scattered and absorbed. Theamount of light reflected back to the photosensor is significantly lessthan before. The recorded image is a grey scale image of thefingerprint.

The single most important problem behind using the concept of FTIR forfingerprint imaging lies in the ability to ensure that the ridge of thefinger completely comes in contact with the optical element. Often, thefinger is very dry and lacks any type of skin oil or moisture. As aresult, when a finger is placed down upon the optical interface, smallair gaps, although microscopic in size, are thick enough to completelyreflect the light back to the photosensor and thus be interpreted as avalley (the space between the fingerprint ridges) instead of a ridge.The net effect is a very spotty image. These spots may eventually bedetected as false minutiae and therefore cause the fingerprint to beimproperly matched against other fingerprints.

In addition to air pockets between the ridge of the finger and the glassinterface being formed by dry skin, two other similar conditions must beconsidered. They are irregularly shaped ridges and sweat pores.Irregularly shaped ridges are those ridges that have nicks and gouges inthem and this type of ridge occurs quite often. This too results in airpockets being formed at each nick and gouge. Sweat pores also present asimilar problem. Sweat pores are tiny openings found along the ridgestructure and appear in the final image when scanned optically. However,when a fingerprint is obtained using an inking process, these sweatpores are filled in with ink and never seen. This results in twocompletely different images of the same finger which causes significantproblems for the image processing software.

Another type of problem occurs if the finger to be imaged is extremelyoily or moist as opposed to extremely dry. In this case, the entireridge of the finger is coated with a thin film of oil. The skin oil withits index of refraction acts very similar to the air pockets caused bydry skin and causes the incident light ray to be completely reflected asopposed to scattered by the irregular surface of the ridge. Here, theentire ridge is unable to be imaged and the print image becomescompletely unreadable. This results from the fact that the thickness ofthe oil or moisture needed to completely reflect the incident light waveis very thin. The actual "thickness" needed to reflect the light isdefined by how deep the light will travel into the second medium beforeit is completely reflected. This depth, known, as the depth ofpenetration, is a function of the wavelength of the incident light usedand the index of refraction (n1 and n2) of the two interfacing surfaces.Referring to FIG. 1, the depth of penetration is given by ##EQU1##

Assuming that the optical element is glass and that the interfacingmedium is air, then for an angle of incidence of 45°, the thickness ofthe air pocket needed to completely reflect a light wave of wavelengthequal to 3 um is given by

    dp=902nm                                                   (2)

Thus, for air pockets of thickness greater than that defined in equation(2), the underlying ridge structure is never seen by the light ray. Ascan be seen from equation (1), the depth of penetration can be alteredby changing the frequency of the incident light, the angle of incidence,or the index of refraction (of the optical element usually via some formof surface coating). This effect is shown in FIG. 2. In fact, all ofthese parameters have been the subject of much research in the hopes ofdefining an optimum set of parameters. Although great improvements wereable to be achieved, this fundamental problem is still a major source ofpoor image quality when optically imaging the finger directly.

Even with the improvements made to this approach, however, there are anumber of problems that affect the overall implementation in a realworld environment when interfacing to large masses of people. The firstof these problems is that the image quality varies significantly betweendry versus oily or wet fingers as previously discussed. This ispartially due to the fact that dry fingers usually result in a thinlayer if air existing between the ridges of the finger and the prism. Inthese cases, it is very difficult to distinguish between ridges andvalleys and the resulting image becomes very blotchy. Furthermore, ithas been documented that the optical system is sensitive to not only dryversus wet fingers, but also smokers fingers versus non-smokers fingers,different skin colors, and the menagerie of dirt, grease and grime thatcan be found on the end of the fingers.

Finally, both of the above mentioned approaches (inking and live scan)suffer from a common shortcoming. It has been found over the years thatcertain occupations cause the ridge structures of the finger to be wornvery thin. Occupations that require the repeated handling of abrasivesurfaces such as banktellers handling money, bricklayers, etc. Once theridge structure is worn fine enough, the optical systems are no longerable to acquire good quality images since the surface structure is noteven there to image. This is a significant shortcoming of theseapproaches since one of the very institutions that could utilize a highsecurity system based on fingerprint identification is the bankingindustry.

However, unlike optics, an ultrasound approach according to the presentinvention offers the ability to image below the surface of the finger.Therefore, if no ridge structure exists or if the ridge structure is toofine to produce a reasonable signal to noise ratio, then sub-dermalfeatures can be used as a means of identification. Any unique sub-dermalstructure could be used for this purpose with special attention beinggiven to the arteries and veins. It is well known that the arteries andveins of the fingers are quite numerous and even different between theleft and right sides of an individual. Therefore, by obtaining an imageof these structures, a person's identity can be established much likeusing the fingerprint. Thus using ultrasound according to the presentinvention provides a means for obtaining these images which, when usedin conjunction with the fingerprint, will produce substantially higherperformance ratios with respect to the accuracy of recognition, i.e.false acceptance rates and false rejection rates.

The basic principle behind the ability to use ultrasound as an imagingmodality is governed by the principles of general wave theory analysisand common to several other modalities in one form or another (i.e.optical, electromagnetic, etc.). The principle is that an acoustic waveupon striking an interface will be partially transmitted and partiallyreflected provided that an acoustical impedance mismatch exists at theinterface. Therefore, by collecting the echoes of the transmitted wave,an overall image of the acoustical discontinuities of the object inquestion can be made. When an ultrasonic beam strikes a surface that issmooth and regular (such as the valley of air found in a fingerprint),the angle at which it is reflected is quite predictable and is referredto as a specular return echo. However, when the beam strikes anirregular shaped surface (such as the ridges of the fingerprint or theblood vessels internal to the finger) the beam is scattered in manydirections and is referred to as a scattered return echo. In the case ofa specular reflector as shown in FIG. 3, the amount of reflection thatis caused by the interface is dependent upon the ratio of the acousticalimpedances of the two interfaces and the angle at which the incidentwave strikes the interface.

It is imperative to understand the reflection and transmission behaviorof ultrasound upon striking a very thin gap such as a small air pocket.For the purposes of this explanation, a small air gap internal to asecond structure shall be used as an example. As an incoming acousticwave of unlimited length strikes the air gap, the wave is split into areflected and transmitted wave. After passing through the air gap, thetransmitted wave is again split a second time. The result is a sequenceof reflections in both directions inside the air gap. On either side asequence of waves leaves the air gap which are superimposed. Theindividual waves are intensified or weakened depending on the phaseposition.

Letting Z₁ represent the acoustic impedance of the the material and Z₂represent the acoustic impedance of air, then the ratio of the twoimpedances can be abbreviated by

    m=Z.sub.1 /Z.sub.2                                         (3)

Defining the thickness of the air gap to be d, then an expression forthe acoustic transmittance D and the acoustic reflectance R is given by##EQU2##

Both expressions are periodical and have a minimum and maximum value asregular intervals as defined by minima of R and maxima of D occuring atd/wavelength=0, 1/2, 2/2, 3/2, etc. and maxima of R and minima of Doccuring at d/wavelength=1/4, 3/4, 5/4, etc. These relationships holdonly for infinitely long waves, i.e. continuous waves. However, in thecase of the very thin air gap, even a short pulse is equivalent to awave train of long duration because the width of the gap is much smallerthan one wavelength. The results therefore apply to pulse transmission.The reflection coefficient R is the ratio of the reflected acousticpressure wave Pr to the incident acoustic pressure wave Pi or R=Pr/Pi,assuming the reflecting interface is infinitely thick (severalwavelengths). The reflection coefficient for very fine air gaps, i.e.thin interface, in any material can be calculated from equation (5). Thesignificance is that if the air gap is thin enough the reflectivity isnear zero. This allows imaging past thin layers of air trapped betweenthe finger and lens which is not possible by the optical approach.Reflection coefficients of 1% are readily measured yet, when viewing thetransmittance, virtually no change is detectable with such a fine gap.

An important component of an ultrasonic imaging system is the probewhich in turn, includes the piezoelectric transducer, the requiredlensing system, mirrors, rotating prisms, etc. The transducerrequirements or parameters are tightly coupled to the specificapplication and for the most part are concerned with resolution andattenuation. These parameters include frequency, quality factor, axialresolution, lateral resolution and focal length.

The selection of the desired operating frequency for the piezoelectrictransducer is determined by the attenuation coefficient of thepropagating medium, depth of penetration, and resolution required forthe particular application. Generally, the limiting resolution (and themore critical one for C-scan imaging which will be discussed presently)is lateral or traverse resolution as opposed to axial or longitudinalresolution as will also be described. The lateral resolution of anultrasonic imaging system is directly proportional to the frequency ofthe system. A good `rule of thumb` is that the maximum resolution thatcan be obtained is on the order of a single wavelength. The wavelengthof ultrasound in water at a frequency of 30 MHz for example, can becalculated as follows: ##EQU3##

Two other influencing factors on operating frequency are the attenuationcoefficient of the propagating medium and the depth of penetrationrequired to obtain the image. There are essentially four causes of waveattenuation in a medium:

1. Divergence of the wavefront

2. Elastic reflection at planar interfaces

3. Elastic scattering from irregularities or point scatterers

4. Absorption.

Many materials, including human tissue, have been empiricallycharacterized with respect to their acoustic attenuation. The measure isa composite of the above mentioned causes of attenuation and is usuallygiven in terms of db/MHz/cm. Table I gives the acoustic attenuation ofsome biological samples at a frequency of 1 MHz. As is easilycalculated, the return signal level of an ultrasonic beam operating at30 MHz in soft tissue and imaging a vessel 1 cm below the surface is:

=(1.5 db)(30 MHz)(2 cm round trip distance)

=90 db

It is very easy to quickly exceed the signal to noise ratio of any highsensitivity receiver used to process the return signal. Thus, apractical limit exists between the required resolution and depth ofpenetration needed for obtaining an image.

                  TABLE I                                                         ______________________________________                                        Acoustic Attenuation at 1 MHz                                                              Attenuation Coefficient                                          Material     (db/cm)                                                          ______________________________________                                        Air          10                                                               Blood        0.18                                                             Bone          3-10                                                            Lung         40                                                               Muscle       1.65-1.75                                                        Soft Tissue  1.35-1.68                                                        Water        0.002                                                            ______________________________________                                    

The quality factor or `Q` of a transducer is a measure of its frequencyresponse about its resonant frequency. The Q of a transducer is directlyrelated to the axial resolution of the system as well as the amount ofradiated acoustic energy. Very high Q transducers have poor axialresolution and radiate small amounts of acoustic power. Thesetransducers are highly efficient however and are usually operated in acontinuous wave (cw) mode as in the case of doppler flowmeters. Low Qtransducers offer very high resolutions and radiate considerably moreacoustic energy into the neighboring medium(s). These transducers areusually operated pulsed mode as in pulse-echo imaging systems.

The Q of a transducer is calculated as the ratio of the resonantfrequency to the frequency width of the half power points as shown bythe curve 10 in FIG. 4 and is given by

    Q=f.sub.1 /.sub.13 f                                       (6)

Another form of the definition of Q in terms of energy is given by:##EQU4##

From this it is readily determined that as the amount of energy radiatedfrom either or both faces of the piezoelectric element increases, i.e.energy lost per cycle increases, then the Q of the transducer decreases.Likewise the converse is also true. Thus, if the goal is to design asystem with a broadband frequency response, then the Q of the transducermust be low. To accomplish this, the mediums interfacing to the faces ofthe crystal must have matching or near matching impedances in order tomaximize the radiated energy. In transducers used for biologicalscanning, the one face of the crystal is generally placed on the surfaceof the skin which represents a much better impedance match than that ofair, thus immediately lowering the Q of the transducer. To ensure thatno layers of trapped air lie between the face of the crystal and thesurface of the skin, often a gel-like substance with an acousticimpedance similar to that of tissue is applied to the skin. Since theacoustic impedance of skin is generally several hundred times greaterthan that of the piezoelectric element, the overall effect on the Q ofthe transducer is minimum.

To lower the Q of the transducer even further, the back face of thetransducer is generally mounted using some type of epoxy with anacoustic impedance much lower than that of air. This will cause energyto be lost through the back face as well as the front face. The overalleffect is that the total amount of power available to radiate into themedium to be imaged has decreased, but this is generally overshadowed bythe improved Q of the transducer. A concern in allowing energy to belost through the back face is that it does not find its way back to thecrystal resulting in some type of standing wave or artifact. Therefore,the epoxy used to mount the element is generally filled with particlesof aluminum or tungsten. This turns the epoxy into a good ultrasonicabsorber and the energy radiated from this face is lost.

Axial resolution is the ability of a transducer to distinguish betweentwo objects spaced in a plane parallel to the direction of beampropagation, also known as the longitudinal plane. As shown in FIG. 5, asingle incident pulse 11 striking a medium 12 with two reflectinginterfaces 13,14 causes two echoes 15,16 back to the transducer. Todetermine the distance between the two interfaces, the total timebetween echoes is measured (dividing by 2 for roundtrip time), andmultiplied by the velocity of sound in that medium. Thus, a measure ofaxial resolution is given by the relationship

    d=tc/2.

From FIG. 5 it can be seen that as the two interfaces 13,14 are movedcloser together, the time between successive echoes decreases.Eventually, as the two interfaces 13,14 are moved close enough together,the time between the two echoes 15,16 will no longer be distinguishable(i.e. the two echo pulses will appear as one long pulse to the receivingelectronics). In order to provide as much separation as possible betweenthe two echoes, it is desirable to have the ringing of the radiatedpressure wave (and hence, the reflected wave) be as short as possible.Therefore, very low Q transducers are used when axial resolution is ofprime importance. This is generally the case when multiple images at avariety of different depths are desired. In this case, the return echoesare only captured during a specific period of time. This time representsthe round trip delay time associated with the particular depth or rangethat is being imaged. This technique is referred to as range gating andwill be discussed in further detail presently.

Lateral resolution is defined as the minimum distance that can beresolved between two points in the transverse plane. This distance isessentially dictated by the size of the beam as measured in the plane inwhich the objects reside. FIG. 6 provides a diagrammatic approach indetermining lateral resolution. Two distinct objects are sonified by abeam 21 of beam width `d` which is swept across the objects. When theobjects are far apart (a distance greater than `d`), two distinct echoes22,23 are returned. Knowing the sweep rate of the transducer andmeasuring the time between the two returns, the distance between the twoobjects can be determined. As the objects are moved closer together, thereturn echoes also move closer together. When the return echoes appearright next to one another, yet still distinguishable, the distancebetween the two objects is the minimum resolvable distance. Thisdistance is defined as the lateral resolution and as can be seen fromFIG. 6, is approximately equal to the size of the spot. Should theobjects continue to move closer together, the individual echoes begin tomerge with one another making the determination of two distinct echoesambiguous. It should be noted however that the ability to detect smallchanges in amplitude of the returned signal will improve the lateralresolution of the system. Many systems often provide an adjustment onthe overall receiver gain and/or sensitivity in order to be able toadjust the system's resolving power.

Naturally, in order to maintain the maximum level resolving capability,the spot size must be kept to a minimum for reasons previouslydiscussed. The size of the beam is smallest at the focal point of thetransducer. Therefore, any objects that are to be imaged should residein a plane located at the focal distance. Often this is difficult to dobecause of a number of application specific problems. The question thenbecomes how much larger does the beam get as it moves away from thefocal point. Another way of stating this is how deep is the regionwithin which the size of the spot falls within certain limits of isoptimum size. The answer to this question is referred to as the depth offocus. The depth of focus is defined as the region surrounding the focalpoint where the spot size is within 1.414/d of its optimum size `d`.FIG. 7 shows the depth of focus for two different transducers.Transducer A has an aperture size equal to that of transducer B.Transducer B however has a much shorter focal length than transducer A.This results in a much small spot size at the focal point (thusproviding better lateral resolution) but diverges much more rapidly asthe distance from the focal point increases. It is easily seen from FIG.7 that the region in which the spot size is within 1,414/d of it minimumspot size `d` is much smaller for transducer B than it is for transducerA. Thus, for tightly focused transducers, the spot size of the beam atthe focal point is much smaller than for weekly focused transducers butthe penalty paid for this is that the depth at which this spot size isable to be held is significantly smaller. Therefore, in applicationswhere several planes of varying depth are needed to be imaged, a weaklyfocused transducer is desirable assuming the spot size is sufficientlysmall enough.

In most high lateral resolution applications, the size of the spotgenerated by the beam from an unfocused piezoelectric element is toolarge. It is this spot size that directly defines lateral resolution asexplained previously. Therefore, to decrease the spot size, a refractivemedium (a lens) or a spherical reflector is used in order to converge asmuch of the beams energy at a specific point in space while, at the sametime, minimizing the amount of energy (sidelobes) everywhere else.

The concept behind an acoustical lens is identical to that for opticallenses and is based on Snell's Law ##EQU5##

A pictoral explanation of this relationship is given in FIG. 8 and canbe stated as the ratio of the sines of the incident ray angle to thetransmitted ray angle being equal to the ratio of the phase velocitiesof the incident medium to the transmit medium. Therefore, by selecting amedium with a phase velocity different from that of its surroundingmedium and by shaping the surface of that medium, an acoustic lens canbe made. It is similar to that which is done in the case of opticallenses with the most significant difference being the fact that for apositively converging (acoustic) lens, a concave face is needed. This isbecause the phase velocity of sound in typical lens material such ascross-linked polystyrene, is greater than that of its surrounding mediumwhich is usually water or tissue. Under the constraints that theacoustic phase velocity of the lens material is greater than thesurrounding medium, FIG. 9 is a diagram of a converging (positive) lens35. The focal length of this lens is given by the expression ##EQU6##where R1 is the radius of the concave surface of the lens, C_(m) is thephase velocity of sound in the surrounding medium and C₁ is the phasevelocity of sound in the lens material. Using this relationship forfocal length and the fact that the radiation pattern from an unfocusedcircular transducer is essentially a Bessel function of first kind(order 1), then the size (diameter) of the dense central portion of thefocused spot from a circular transducer can be calculated as

    d=2.44(1.sub.f /D)(wavelength)                             (10)

where D=transducer diameter. The constant 2.44 defines the distancebetween the first zeroes of the main lobe of the Bessel function.However, this relationship is quite conservative and often the 6 dbpoints are used to define the size of the focused spot. Severalinstruments provide a manually adjustable gain to allow even moreimproved lateral resolution at the cost of system SNR (Signal-to-NoiseRatio). The relationship for spot size is often approximated as

    d=(1.sub.f /D)(wavelength)                                 (11)

As in optics, the ratio of the focal length to the beam diameter isreferred to as the `f` number of the lens. Generally, `f` numbers lessthan 1 are considered difficult or too costly to fabricate.

In using lenses for specific applications, an effect known as sphericalabberation must be accounted for. In order to fully understand sphericalabberation, one need only to look at Snell's relationship along withFIG. 9. It can be seen that for rays far away from the center of thelens, and assuming all rays are parallel to each other and perpendicularto the plane face of the lens, the angle at which the rays strike thespherical face of the lens increases. As the angle increases, the ratioof the sines varies slightly. The variance causes a shift in the focallength which in turn causes an increase in spot size or a smearingeffect. The effect of this smearing and whether or not it needs to becorrected for is application specific. However, it is relatively simpleto correct for this effect by making the face of the lens elliptical asopposed to spherical.

Another concern in using lenses to focus a beam lies in the inability toimage objects that do not reside in the plane of focus. As FIG. 7 shows,the spot size of a beam rapidly diverges on either side of the focalplane. The rate of divergence increases as the focus spot size getssmaller. The axial distance over which the beam maintains itsapproximate focused size is given by

    Depth of Focus=3(1.sub.f /D).sup.2 (wavelength)

and is referred to as the depth of focus. Therefore, using a tightlyfocused spot to improve lateral resolution decreases the overall depthof focus. This in turn makes it difficult to apply certain types ofelectronic processing such as range gates due to the fact that lateralresolution outside the plane focus is severely compromised.

The system of the present invention employs a probe which comprises allthose elements other than the interface electronics that are needed toperform a scan. This includes the piezoelctric transducer, the focusinglenses, spinning mirrors, etc. The previous description explained thetransducer parameters being tightly coupled to the characteristics ofthe image to be acquired. The following description will show that theprobe architecture is tightly coupled to the application requirementsunder which the image is to be acquired. Considerations such as speed ofacquisition and overall scan area all factor into the type of probearchitecture that is chosen. For the purposes of illustration, a varietyof different architectures will be described which will enable a clearerunderstanding of the reasons for providing the particular probearchitecture used in the system of the present invention.

Probes using a single element piezoelectric crystal for both driving andreceiving the acoustic energy are functionally similar to the probe ortransducer 50 shown in FIG. 10. In particular, transducer 50 comprises agenerally cylindrical housing 52 closed at one end and having anacoustic lens 54 similar to lens 35 of FIG. 9 mounted in the housing atthe opposite open end thereof. A piezoelectric crystal 56 is adjacentthe planar surface of lens 54, and a body of acoustic absorbing material58 is in housing 52 and in contact with lens 54. Crystal 56 is connectedto external pulsing and receiving circuitry (not shown) via leads 60,62and an electrical connector 64 on the outer surface of housing 52.Probes of this type generate a fixed focus, non-steerable beam patternas used to perform an A-mode scan which will be described. In order toachieve other scan geometries using this type of probe, some form ofmechanical motion is required. This motion can be either a wobbling orrotating of the crystal, the use of rotating mirrors to deflect theacoustic beam, or physical movement of the entire probe assembly as willbe described. Naturally, whenever any type of mechanical movement isintroduced into a system, wear factor or mean time between failurebecomes an issue. Furthermore, most of the imaging systems that are usedfor medical scanning are required to be real time (ie. video framerates). To scan an area of any dimension at these rates using mechanicalmotion is a difficult task and usually involves high speed, highperformance miniature motors. These motors which are responsible forcreating the mechanical movement needed to perform the scan alsogenerate a fair amount of vibration and noise which may or may not be aproblem in certain applications.

The most important advantage of the single element probe architecture isthe simplicity of the pulse/receiver electronics. Since there is only asingle element to interface to, only one copy of the pulser/receiverelectronics is required. This is in contrast to the multielement probeswhich could easily have up to 100 or more elements resulting in costlyinterface electronics.

The type of image to be acquired and the nature of image informationneeded will determine the proper scan geometry to be used. In manyinstances, the differences between the individual geometries are subtle,while in other cases the differences are quite significant. The probearchitecture and scan geometry are somewhat correlated. Therefore, thetype of scan required will most probably influence the choice of probearchitectures to be used.

Turning now to various types of scans, in A-mode or amplitude-mode ofoperation a single element probe is excited with a very short electricalpulse. This pulse causes the low Q transducer to vibrate for a shortperiod of time thus radiating acoustic energy into the sample to beimaged. As the sound wave reaches an interface with a different acousticimpedance, a portion of the signal is reflected back towards thetransducer while a portion continues onward. Each time the sound wavestrikes a new interface, part of the signal is returned and partcontinues to propagate forward.

After the initial transmit pulse has been applied to the transducer, theinterface electronics are then switched into receive mode. The amplitudeof the echoes returned from the various interfaces are then displayedagainst a horizontal axis representing time. Observing the total elapsedtime of the individual echoes (and dividing by 2 to compensate for roundtrip propagation delays), the depth of the particular interface can bedetermined. Often an A-mode scan is run prior to trying to obtain animage using a B-scan or C-scan. The depth information returned from theA-scan can be used to set up appropriate range gates, alter focal lengthin the case of dynamically focused systems, or vary the gain of thereceiver at the appropriate time thus emphasizing the region ofinterest.

B-mode or brightness mode scanning can be thought of as multiple A-Scansplaced side by side where the amplitude axis of the A-scan is nowdisplayed as a brightness value or grey scale on the CRT. The scan headis usually (but not always) held in a fixed position and a sector isswept out and displayed on the CRT. The image displayed is a sectorwhose structures vary in depth from the most shallow structures to thedeepest structures. This results in a cross-sectional view of the regionbeing scanned.

This mode of imaging is widely used in the medical community. Often,however, the reflections from structures in the body result inreflections that are somewhat specular in nature. As a result, if thisstructure does not lie at an angle close to perpendicular to thepropagation path of the beam, the reflection will be missed by thetransducer. It is therefore often quite advantageous to view a structurefrom several directions in order to create a composite image of thestructure. This is accomplished by fixing a mechanical sensing systemonto the arm of the B-scan head. The positioning system keeps track ofwhere it is in relationship to the patient's body and feeds thisinformation back to the computer. This allows multiple images to beacquired from a variety of angles and properly reconstruct the imageinternal to the computer resulting in a much higher definition image.This type of B-scan is referred to as compound B-scan.

There are two distinctively different definitions of C-modeultrasonography. The medical community generally refers to this type ofimaging as through-transmission. Two transducers are used in apitch-catch configuration. Since there is usually no clinicalinformation to be obtained from a C-mode scan, the medical communityrarely uses this type of scan geometry.

Another and more widely accepted definition of C-mode ultrasonography,as used in the non-destructive test industry, is a parallel beampropagation used to capture an image along a plane perpendicular to thepropagation path of the ultrasound. The entire image that is createdlies in a place at a constant distance from the transducer. As a result,the scan geometry can be easily optimized so that the image planeresides at the focal point of the transducer thereby producing the bestpossible image with respect to lateral resolution.

A C-mode scan can be performed by collimating the rays of a B-mode scan,performing range gating on the B-mode scan to look at a particulardepth, and stacking several B-mode scans together to generate thedesired two-dimensional image.

FIG. 11 illustrates the system 100 according to the present invention.System 100 performs a C-scan over a fixed area, for example0.500"×0.750", and applies range gating on this area anywhere from thesurface down to a given depth, for example 5 millimeters. This scangeometry is accomplished by essentially performing a collimated B-scanwith range gating and then stacking the output of multiple range gatedB-scans together to obtain a C-scan geometry and the desired image. Oneof the principal components of system 100 is a probe 102 which, in turn,includes a fixed focus, 30 Mhz transducer used to create a spot size ofapproximately 0.006" in diameter which defines the lateral resolutionlimit as previously described. Probe 102 will be described in detailpresently. This spot is reflected off an acoustic mirror in probe 102towards the object to be imaged. The mirror is attached to a rotatingmotor which provides a constant scan speed. As the mirror rotates, thespot is swept across the object in a sector like fashion. In order toeliminate the varying depths of focus caused by a sector scan, probe 102also includes a lens to collimate the beam. Accordingly, as the acousticmirror is rotated by the object to be scanned, a linear scan line isgenerated in which range gating can be applied. Once an entire line hasbeen scanned, the transducer and mirror assembly motor is stepped by0,006" using linear actuator to prepare for the next scan line. Thisentire process is repeated until an area equal to 0.500"×0.750" has beenscanned. The detailed structure and operation of probe 102 will bedescribed presently.

System 100 further comprises a pulser/receiver 104 operatively connectedto probe 102. The data returned from the ultrasonic transducer isamplified by pulser/receiver 104 and passed to high speed signalprocessing means generally designated 106. By way of example,pulser/receiver 104 has a bandwidth of 40 Mhz and is able to amplify thereturned data signals by 40 db. Signal processing means 106 includes alog amplification and peak detection circuit portion 108, asynchronizing and timing circuit portion 110 and a digital range gatelogic circuit portion 112, all of which will be described. The output ofthe high speed signal processing circuitry 106 is transmitted along path114 as 8 bits of digital data and stored internal to computer 116 whereit can be displayed on a 256 grey scale monochrome monitor or stored ondisk for viewing at a later time. Computer 116 is not only responsiblefor collecting, storing, and displaying the data, but also controls theentire data acquisition process via the execution of control software118. This software is responsible for initializing/controlling thehardware 110, issuing step commands to a stepper motor controller 120associated with probe 102, and providing the necessary user interface toinitiate the entire scan process. Signals containing digital status andcontrol information are transmitted between computer 112 and signalprocessing means 106 via path 122 and 124. A high voltage d.c. powersupply 126 is associated with pulser/receiver 104.

It is apparent that the step size along one axis of scan motion, i.e.the `x` axis, is directly controlled by the size and number of stepsthat are delivered by the stepper motor of the linear actuator in theprobe. Furthermore, since the commands to cause a step are generated bythe software running on computer 116, there is no problem in determiningwhat the linear actuator stepper motor is currently doing since it isunder complete control of the software. Determining the position of therotating acoustic mirror in probe 102 also is straightforward since ittoo is controlled by software.

Once the position of the rotating mirror of probe 102 has beendetermined, the pulse repetition frequency of pulser/receiver 104 can beadjusted to provide the required step size in the other axis of scanmotion, i.e. the `y` axis. This step size is determined by the amount ofrotation the acoustic mirror of probe 102 moves between the main bangpulses of pulser/receiver 104. By way of example, for a 600 rpm rotationrate and knowing the distance that the acoustic mirror is spaced fromthe collimating lens (the focal length), to sweep out a distance of0.006" per pixel requires 100 microseconds of rotation. Thus, the pulserepetition frequency or PRF of the pulser/receiver must be set to 10Khz. FIG. 12 shows this PRF timing including the main bang pulses 130 ofpulser/receiver 104, the return echo 132 and the range gate pulse 134.

Probe 102 is shown in further detail in FIGS. 13 and 14 and includes asingle 30 Mhz, fixed focus transducer 140 mounted internal to an overallhousing 142 which is fixed to a base 143. Housing 142 includes a cavityor chamber 144 filled with water and fixed in space. Transducer 140 iscarried by a linear slide rail assembly comprising guide bars 146, siderails 148 and brackets 150 and 152 which is controlled by a linearactuator 154 comprising a stepper motor and a ball screw as is wellknown to those skilled in the art. In particular, transducer 140 ismounted in bracket 150. This allows the transducer 140 to be moved inboth directions along one axis of scan motion. This axis of motion willbe referred to as the `x` axis. A flexible bellows 156 is open to cavity144 and sealingly connected to housing 142 and bracket 150. The secondaxis of scan motion, the `y` axis, is achieved by a rotating acousticmirror 156 that also resides internal to the probe housing 142 and isfixed in space with respect to the ultrasonic transducer. The mirror 156is mounted on the end of a shaft 158 rotatably mounted in housing 142and driven by a stepper motor 160 carried by bracket 152. Motor 160 andtransducer 140 are moved together linearly along the `x` axis. Thus, asthe transducer 140 moves along the one axis of the probe housing 142,the acoustic mirror 156 also rotates at a rate of 600 RPM. This rotationcauses the ultrasonic beam to be deflected off axis according to thepresent position of the acoustic mirror. The net effect is that a secondscan motion, orthogonal to the motion caused by the linear actuator 154,is created. Therefore, to create a two dimensional scan image, the databeing returned along the `y` axis as a result of the rotating acousticmirror 156 is collected and stored for a single scan line. Once thisscan line is complete, computer 116 issues a command to a controller(not shown) for linear actuator 154 to step to the next scan line in thex-axis. After the step has been performed, computer 116 beginscollecting data from a second scan line in the y-axis. This processrepeats for the entire scan image.

By way of example, in an illustrative system transducer 140 is a singleelement, 30 MHz, fixed focus transducer having a structural arrangementsimilar to that shown in FIG. 10. The aperture size of the transducer is5/16" with a focal length of 0.750". This yields an `f` number ofapproximately 2.4. A lens is used to focus the beam at the focal lengthof 0.750". This results in a very tight spot at the focal point of about0.006" (measured at the -6 db points). It is this spot size thatdetermines the lateral resolution limit. In addition to high lateralresolution, good axial resolution is also required for accurate rangegating. The Q of the transducer must therefore also be kept very low.This particular transducer has a Q of 2.

By way of example, in an illustrative system, linear actuator 154 isavailable commercially from Haydon Switch & Instrument Inc. under thedesignation Series 35800 and stepper motor 160 is commercially availablefrom Vernitech under the designation 90 Stepper Motor 08NPD-AA4-A1.

The rotating acoustic mirror 156 is used to sweep out a single scan linein the y-axis. The returned data is then range gated and stored as asingle scan line in the overall image. By way of example, in anillustrative system, plate glass pitched at a 52° angle is used as theacoustic mirror 156. There are several factors that influence theselection of the type of mirror material, its thickness, and the angleat which to place it. First and foremost, the probe architectureconstitutes the requirement for deflecting the beam at right angles (ornear right angles) to its original propagation path. Thus, thereflecting surface must be pitched at or about 45° with respect to theoriginal path of propagation. Secondly, in order to maximize the amountof reflected compressional wave from the surface of the mirror 156 theamount of compressional wave that is transmitted into the glass must beminimized. In order to do this, the incident wave must strike thesurface of the mirror 156 at the critical angle in order for modeconversion (from compressional to shear) to take place. It is alwaysdesirable to operate slightly beyond the critical angle in order toensure complete conversion. Therefore, a material must be selectedhaving phase velocity and acoustic impedance such that mode conversionoccurs at an angle slightly less than 45°. Ordinary plate glass has sucha critical angle when placed in water. As a rule of thumb, the thicknessof the reflecting material should be at least 2-3 wavelengths to ensurethat the material does not vibrate but reflects the beam.

Using an acoustic mirror angled at 45° results in propagation pathsorthogonal to the walls of the collimating lens as well as the walls ofthe probe body itself. These walls create reflections themselves andfurthermore, the reflections occurring from the lens may cause artifactsto appear at the depth of the desired range gate. Therefore, toeliminate these artifacts, the mirror 156 is pitched an additional 7°.Since the walls of the lens are specular reflectors, the reflectedsignal is now angled sufficiently away from the acoustic mirror so as tomiss it altogether. As a result, no unwanted reflections from any of thesurfaces of the probe appear thus eliminating the potential for anyartifacts to occur. This naturally, however, results in a majordifference with respect to the type of object that can be scanned.Specifically those objects that appear as specular reflectors result inreturns that are missed by the acoustic mirror 156. Only objects thatappear as a scatterer, that is return the signal in all directions, areable to be imaged. Should specular images be desired, then the 52 mirrorcan be replaced by a 45 mirror.

The rotating acoustic mirror 156 is responsible for sweeping out asector scan along the y-axis. The characteristic of a sector scan is aspherical field of focus in the longitudinal plane. That is, without anycorrection from external lenses, the distance to the focal point at theedge of a sector is different than the distance to the focal point atthe center of the sector when measured from the object being imaged.FIG. 15 shows the geometry of the field of focus 170 for an uncorrectedsector scan. Since lateral resolution is of the utmost importance forthis particular application, it is essential that the object beingscanned be in focus for the entire scan area since it is at thisdistance that spot size is held to a minimum. This is compounded by thefact that depth of focus is very shallow due to the tight focusingemployed to reduce the spot size. Therefore, to alleviate this problem aplano-concave lens designated 172 in FIGS. 13 and 14 is added betweenthe rotating acoustic mirror 156 and the object, such as a finger, to beimaged. One function of the lens is that it collimates the beam. Thecharacteristic fan geometry of the sector scan is converted to theparallel geometry associated with C-scans. The result is that on the farside of the lens 172 a single plane 174 exists in which the focal pointof the transducer resides as shown in the diagrammatic view of FIG. 16.This plane can be varied in its position along the longitudinal `z` axisaccordingly by repositioning the transducer with respect to the rotatingmirror 156. Therefore, sub-surface scans can be optimized with respectto lateral resolution by moving transducer 140 closer to the rotatingmirror 156 thereby moving the focal point of the transducer deeper intothe structure being scanned. However, deeper scans can also be obtainedby simply changing the electronic range gate without any probeadjustment at all. This is provided that the effects of the depth offocus are tolerable, i.e. increased spot size.

A second function of lens 172 is to provide a natural interface for theobject being imaged (the finger) to be placed upon. If the collimatinglens were not there, some other form of interface would have to beprovided in order to support the finger being imaged. Lens 172 not onlysteadies the finger while it is being scanned, but also tends to flattenthe finger out on the edges, thus creating a larger plane to be imaged.

A number of materials can be used as the material for lens 172. However,in order to maximize the quality of the image, a material havingacoustic impedance closely matched to that of the skin is highlydesirable. This causes the ultrasound on the ridges to be absorbedreasonably well while the ultrasound at the valleys (air) is reflectedalmost completely. Yet, the material selected must still exhibit anadequate phase velocity difference from water such that the physicalsize of the lens is reasonable. Cross-linked polystyrene appears to be avery good choice for lens material when imaging the body. Its acousticimpedance and phase velocities are not only suitable for this type ofapplication, but it also is a readily available, easy to work with, andan inexpensive material.

FIGS. 17 and 18 provide a ray trace of the ultrasonic beam as it strikesa scatter reflector and specular reflector respectively. Acoustic mirror156 is on the end of shaft 158 driven by motor 160 of FIGS. 13 and 14.As shown in FIG. 19, the angle of the incoming incident ray 184 issufficient enough so as to cause the specular return to miss rotatingacoustic mirror 156, thereby never returning it back to transducer 140.As previously described, the incident angle of the ultrasonic ray uponcollimating lens 172 is created by the pitch of acoustic mirror 156(which is 7° beyond normal). Performing some simple geometriccalculations shows that the reflected beam 186 does indeed miss acousticmirror 156, thereby not allowing any energy to be returned to transducer140. FIG. 19 shows a typical ultrasonic return as seen by transducer 140when echoing a specular reflector i.e. an object such as a fingerprint.In particular, the main bang pulse from pulser 104 is designated 190,the return echo pulse is designated 192 and the range gate pulse isshown at 194. The output of transducer 140 eventually gets digitized andsent back to computer 116 for display. Since the return amplitude 192 isessentially zero, the corresponding grey scale value will also be zero.This results in a black or dark region on the monitor. The valleys ofthe fingerprint are essentially air and therefore the plastic-airinterface causes a specular return. Since specular returns are displayedas dark regions, the valleys of the fingerprint will also be displayedas dark regions.

The ridges of the finger appear as ultrasonic scatterers. That is, theultrasonic energy is reflected back in all directions. Provided thatsome of the rays are reflected back at a angle which compensates for theincident angle (7°), the return echoes will be seen by transducer 140,thus resulting in a return 196 similar to that shown in FIG. 20. Thisrelatively large amplitude return 196 eventually is passed to computer116 as a large grey scale value which in turn is displayed as a white orvery light region on the monitor. Therefore, all ridges of thefingerprint are displayed as very light or bright regions. Once thebasic image has been captured internal to computer 116, should aninversion of the image be desired, i.e. all valleys appear as brightregions while all ridges appear as dark regions, it is quitestraightforward to perform this inversion in software.

An important observation can be made in the case of the scatter return.Since by definition the scatter reflector causes energy to be returnedat all angles, an equal amount of energy is capable of being collectedat various distances away from collimating lens 172. These varyingdistances are simply the focal points of the lens for return echoes ofdifferent angles. This characteristic is an important one from the pointof view of fabricating the probe assembly. First, it allows for theplacement of rotating acoustic mirror 156 to be non-critical, since anequal amount of energy is capable of being collected at any distance.Secondly, the quality of collimating lens 172 also becomes non-exacting.Specifically, the effects of spherical abberation are no longer an issuethus allowing a spherical lens to be fabricated.

FIG. 21 illustrates a preferred form of circuit for implementing thesystem of FIG. 11. Power is provided to all of the subassemblies of thesystem using an off-the-shelf DC power supply 200. This supply providesfour rail voltages +5, -5, +15, and -15 which are sent to theappropriate subassemblies. Should voltages other than these be requiredby any one subassembly, then these voltages are derived on thatsubassembly from one of the four main rail voltages. The four rails, +5,-5, +15, and -15, are capable of supplying 1.5 amps., 1.5 amps, 0.8 ampsand 0.8 amps respectively.

The high voltage DC power supply 126 is for driving the piezoelectricoscillator in transducer 140. A 115 VAC input 210 is applied to theprimary of a 1:2 turns ratio transformer 212 in order to double the ACvoltage output. The secondary of transformer 212 is connected to a fullwave rectifier 214, the output of which is passed through a voltagedoubler circuit including capacitor 218. The output of the voltagedoubler 218 is applied to a series regulator 220, the control of whichis provided by a zener diode 222 establishing a reference voltage to thebase of a series pass transistor 224. The zener voltage is first passedthrough a potentiometer 226 to allow the high voltage output to bevariable from 50 to 300 VDC. This output is used by the pulser/pre-amp104 to charge a capacitor used to drive the transducer 140.

The pulser/pre-amp 104 is responsible for sending driving signals to andreceiving signals from the transducer 140, amplifying the receivedsignals, and passing the amplified signals to the log receiver 108. Acapacitor 230 in the pulser/pre-amp subassembly 104, which is connectedto transducer 140 via line 232, is initially charged to full value bythe HVDC supply 126. The charged capacitor is rapidly discharged intothe piezoelectric crystal of transducer 140 by means of a high speedavalance transistor 234. In particular, switching of transistor 234reverses the polarity of voltage on capacitor 230 thereby applying anegative spike or discharge pulse to transducer 140. Falling edges of0.8 nanoseconds over a 300 VDC range are obtainable. The high transmitvoltage spike is suppressed by a transmit/receive switch 236 comprisingtwo back-to-back diodes. This switch limits the high (300 VDC) voltagespike due to the transmit pulse from reaching the preamplifier and thusdamaging it. The low level ultrasonic returns are passed umimpeded andare amplified by the preamplifier comprising a high input impedance FETamplifier 240. This amplifier provide approximately 40 db of linear gainto the signal which is then buffered by a line driver 242 and sent tothe log receiver subassembly 108.

The log receiver subassembly 108 includes an input amplifier 250 whichbuffers the input signal from the pulser/pre-amp 104, and apotentiometer 252 provides a variable attenuation in order to bring thesignal into the proper range of the log amplifier. The log amplifiercomprises the combination of operational amplifier 254 and feedbacktransistor 256. The signal is logarithmically amplified therein by 60 dband again passed to a variable attenuator in the form of potentiometer260. Attenuator 260 is used to scale the output of the log amplifierwhich is then sent to a video amplifier 262 for final scaling. Theoutput of the video amplifier is a signal that swings between +2 vdc and-2 vdc which is the scaling that is required by the peak detectorcircuitry in the A/D convertor subassembly 270.

The A/D convertor subassembly 270 is responsible for providing peakdetection on the output signal of the log receiver subassembly 108 aswell as generating all of the necessary timing for issuing main bangpulses and performing range gating. The output of the log receiver ispassed through a buffer 272 and sent to a peak detector circuit 274.This circuit 274 is capable of detecting a peak of 10 nanoseconds induration with an amplitude of only 7 millivolts, and circuit 274 can beimplemented by various arrangements well known to those skilled in theart. For example, in an illustrative circuit peak detector 274 iscommercially available from TRW LSI Products Inc. under the designationMonolithic Peak Digitizer TDC1035. The output of peak detector circuit274 is 8 bit digital data which passes through a TTL line driver 276 andis sent to comparator 116 for reading. By way of example, in anillustrative system, computer 116 is an Apple MAC II. A timing pulse isapplied via line 280 to the peak detection circuit 274 to define therange gate. All of the timing is initiated from a free running 555 timer282. The output of timer 282 is responsible for the generation of threeother timing pulses as follows.

The first timing pulse is the main bang pulse used to by thepulser/pre-amp subassembly 104 to discharge capacitor 230 into thetransducer 140. The timing pulse from timer 282 is adjusted in widthusing a one shot 286. The output of one shot 286 is then sent to a 50ohm line driver 288 and driven at a 15 volt level where it is receivedby the pulser/pre-amp 104.

The second timing pulse is the generation of the range gate used by thepeak detector circuit 274. This pulse is formed by a series of oneshots. The first one shot 292 adjusts the total delay of the range gatewith respect to the main bang pulse, while the second one shot 294determines the pulse width or width of the actual range gate. Thus, byadjusting these two one shots, the depth and size of the range gate canbe adjusted. Typically, delays of approximately 20 microseconds andwidths of about 1 microsecond are employed.

The third timing pulse is the generation of the data available pulseused by computer 116 to indicate that the data on the output of the peakdetector 274 is in fact valid data. This pulse is initiated by thetrailing edge of the range gate pulse on line 280 (which is initiated bythe main bang pulse) and then delayed an appropriate amount through theuse of one shot 296. The width of this delayed signal is then alteredusing a second one shot 298. It is the signal which is sent to a TTLbuffer (not shown) and passed on to computer 116 as a DATA AVAILABLEpulse.

The computer 116 is essentially waiting for the issuance of a DATAAVAILABLE pulse by the hardware. Once it detects the pulse, it reads thedata present on the output of the peak detector 274 and stores it. Thisprocess is described in more detail further on as well as the control ofthe overall probe assembly 102.

In order to collect data echoed from the object being imaged as opposedto random returns that may be echoed from the probe body itself, it isnecessary to provide computer 116 with information on the rotationalposition of acoustic mirror 156. The data acquisition process must occurduring the interval when mirror 156 is facing collimating lens 172, i.e.facing the object being imaged. In order to determine when this intervalis occurring, the position of motor shaft 158 and therefore acousticmirro 156 must be determined. This is accomplished by providing anencoder arrangement (not shown) associated with motor shaft 158 forproviding information signals for use by computer 116.

In particular, a marker (not shown) such as a small piece of blackmaterial is placed on motor shaft 158 at a known angular positionrelative to the rotation of shaft 158 and the angular position of mirror156. A combination infrared light-emitting diode and photodetector (notshown) is positioned so as to generate an output pulse each time themarker is in the field of view of the photodetector, which occurs onceper rotation of shaft 158. The foregoing is typical of encoderarrangements well known to those skilled in the art.

The encoder output signal is transmitted from probe 102 via line 310 toone input of a comparator 312 in A/D converter 270. Potentiometer 314 isconnected to the other input of comparator to establish a threshold. Asa result, only signals above the pre-set threshold will be passed bycomparator 312 in a known manner so as to distinguish the photodetectoroutput from spurious signals. The output of comparator 312 thus is asynchronization pulse which indicates the position of mirror 156. Adigital time delay is added by one shot 318 to mark when mirror 156 isat the beginning of its scan of the collimating lens 172. The width ofthe delayed pulse is altered by one shot 320 and the resulting signal issent to computer 116 via line 322.

The software designated 330 in FIG. 22 controls the data acquisitionprocess on the subsystem of computer 116. The software is responsiblefor interfacing and controlling the parallel digital I/O ports found onthe data acquisition board of computer 116. It is entirely through thedata acquisition board that computer 116 communicates with the signalprocessing means 106. The software 330 initializes the hardware whenrequired and keeps track of where the data acquisition process is withrespect to the entire run in order to control the stepper motor stepfunction and direction appropriately. Therefore, the software needs onlyto execute at `moderate` throughput rates in order to keep up with therequired processing tasks. The programming language BASIC was chosen forthe language in which to implement the system. The compiled code forthis language provided more than enough processing power to handle thetasks fast enough and yet was a simple and easy language to write anddebug thereby enabling fast and accurate coding to take place. Aflowchart of the overall programmatic flow is given in FIG. 22 and thedetailed code is presented in the attached Appendix. A timing diagram ofthe overall system activities as defined by the Control software isgiven in FIG. 23.

Referring now to FIG. 22, in the initialization routine 332 theinitialization code is involved upon power up or upon restart of amultiple scan. It is this code that is responsible for initializing allprogram variables as well as hardware parameters found on the dataacquisition portion of computer 112.

The main driver routine is responsible for invoking all of the supportsubroutines at the appropriate time in the appropriate sequence.Furthermore, service oriented questions to the user such as "Do you wishto run another scan", "Do you wish to store the scanned data", or "Enterthe file name in which to store the data" are all output as part of thisroutine. These are the only three responses that the user will have tomake in order to run a scan. The first question, "Do you wish to runanother scan", is answered either a `y` for yes or `n` for no followedby a carriage return. This allows the user the option to either quit theprogram in an orderly fashion, or continue on to the next stage ofprocessing which is to actually perform a scan.

The second question, "Do you wish to store the scanned data", appearsimmediately after a scan has been performed. Again this question isanswered with either a `y` for yes or `n` for no followed by a carriagereturn. This allows the user the option of creating and storing a fileon hard disk with the raw (unprocessed) data. Conversely, there may becertain reasons why the user suspects (or knows) the data to be invalid,therefore not desiring to store it. The response of `n` will circumventthe storage routine call and return back to the start of the program.The third question, "Enter the file name in which to store the data",appears only if the user has responded affirmatively to the secondquestion. If so, a maximum 27 character file name must be enteredfollowed by a carriage return. This is the actual name of the file as itis to be stored onto the hard disk.

After the software and hardware has been initialized, the main routinepasses program control to the HldUntlRqst subroutine 334. The purpose ofthis routine is to wait until the user signifies that a scan shouldcommence now. While the routine is waiting for a request to scan fromthe user, a "Waiting for Request" message is output onto the CRT. Once arequest has been made, control is returned to the main driver routine.

A request to scan can be initiated by either activating an externalRequest to Scan switch that drives bit `0` of Port `A` or typing theletter `R` on the main keyboard. Therefore, this subroutine monitors bit0 of Port A on the data acquisition board. Should this line go low, or arequest be made by the user via typing the letter `R` on the keyboard,then the subroutine interprets this as a request to scan and returnscontrol back to the main driver for further processing. Bit 0 of Port`A` is driven low only by an external toggle switch that is mounted onthe main electronics assembly and activated by the user when a requestto scan is desired. The `OR`ing of the keyboard response with theexternal switch is merely for purposes of convenience.

Once a request to scan has been initiated by the user, the softwarepasses control to the HldUntlSync subroutine 336. This routine monitorsbit 1 of Port `A` to determine if a sync pulse as driven by the hardwarehas occurred. This would then tell the software to prepare for dataacquisition. Since the detection of the sync pulse is quite critical andvendor-supplied routines for digital I/O are extremely slow, individualPEEK commands are used to interrogate the hardware. They operatesignificantly faster than the general purpose routines provided with thedata acquisition board.

While the routine is waiting for the detection of a sync pulse, a"Waiting for sync" message appears on the CRT. Under normal operatingconditions, a sync pulse occurs within one revolution of the acousticmirror 156 and as a result, the signal is seen immediately and nomessage is output.

Turning now to the AcqEcho subroutine 338, the start of this process isnot critical due to the fact that the actual data acquisition timing iscontrolled by appropriate timing pulses driven by the signal processingmeans 106. This subroutine is invoked by the main routine once the syncpulse has been detected. The routine starts the data acquisition processand then continually monitors that process for successful completion.Once completion has been determined, the data is stored in an array andcontrol is returned back to the main driving routine. The index to thearray is not reset but allowed to continue to increment. Upon completionof the entire scan area, Echoarray& is a 256×192 array containing thescanned data.

The actual amount of scanned data that is taken by the system is 256×192points. In order to store a more uniform file, however, the data ispadded with an additional 64 rows to give an even 256×256 pixels. Theactual scanned data lies in the center of the stored (and displayed)data and the additional 64 rows are divided into 32 each before andafter the scanned data. A grey scale value of 256 is given to the filldata. This results in an all white area to be displayed on themonochrome CRT.

After an entire line has been scanned in the y-axis, the stepper motor160 and thus mirror 156 must be positioned to the next line in thex-axis in preparation for the next scan. This positioning isaccomplished by the step motor subroutine 340 which refers to the stepmotor included in linear actuator 160. This subroutine is invokedeverytime the main routine detects the fact that an entire line has beenscanned. The subroutine then issues a series of pulses to the motorcontroller by toggleing bit 2 of port B on the data acquisition board.The amount of pulses delivered to the motor controller is such that theresultant linear motion as seen by the probe assembly is approximately0.006". Thus, should an increase or decrease in the lateral `x`resolution be desired, then the number of pulses delivered to the motorcontroller as defined by this subroutine must be modified.

An image is comprised of 192 individual scan lines. The Updateysubroutine 342 is responsible for keeping track of how many scan lineshave been scanned already. Once a count of 192 has been reached, thereturn code from the subroutine is such that the main program realizesthat the scan is complete and it is time to rewind the motor 160.Furthermore, it is the responsibility of this subroutine to reset itsinternal counter to zero in preparation for a new scan. The variablemtrptr% is used to keep track of the count and is compared to a constant192. Therefore, should a larger size scan in the y direction berequired, then this constant must be changed or replaced by a variablethat is initialized as part of the program initialization procedure.

Once the entire scan has been performed, software control is passed tothe rewind motor subroutine 336. The purpose of this subroutine is toposition the stepper motor 160 back to the original starting point inpreparation for another scan. Since the actual number of scan lines thatthe stepper motor 160 moves is 192, the motor must be stepped in thereverse direction 192 steps. However, there is a certain amount ofmechanical flex that can be found in the system with respect to thepositioning system. The first several steps made by the stepping motor160 which is a direction opposite to that which it has been traveling,do not cause the probe 102 to step in that direction but rather are usedto take the backlash out of the system. In order to position the systemback to its true starting point in preparation for a new scan, thestepper motor 160 must be rewound past the point at which it started andthen stepped forward to its actual starting point. In doing so, theeffect of the mechanical backlash of the system is eliminated (or atleast minimized). Therefore, when the software issues the first realstep command during a scan, the command will actually cause the probe tomove, as opposed to simply flexing the entire assembly.

The Storeit subroutine 346 is responsible for scaling the scanned dataappropriately, formatting it and storing it onto hard disk under a userdefined file name. This routine, due to the amount of data it musthandle as well as the extensive I/O that it must perform, is arelatively time consuming process. It cannot be called during the actualdata acquisition process since it would cause the sync timing pulse tobe missed. Therefore, this subroutine is invoked by the main programonce the entire scan process has been completed. It is theresponsibility of the main driving routine to query the user if thescanned data is to be saved or not and if so, under what file name. Thisfile name is then passed to the Storeit subroutine where an empty fileis opened up on the hard disk and the data is stored.

The timing diagram of FIG. 23 shows the relationship between the requestto scan 350, synchronization 352, line scan 354 and step motor 356pulses, respectively.

In view of the foregoing, the present invention is directed primarilytoward the imaging of surface topology using ultrasound, with specificemphasis on the application to fingerprint imaging. However, the systemof the present invention is capable of scanning any surface topologyother than fingerprints so long as the acoustic impedance mismatchbetween the object to be scanned and the surface of the lens is closeenough to allow part of the ultrasound wave to propagate into thematerial. Therefore, for certain materials, an acoustic coupling agentmay be highly desirable. Such coupling agents come in many forms for useon many different materials. In addition, the ability of the system ofthe present invention to scan beneath the surface of an object presentsa capability not found in optical systems. In the case of performingpersonal identification, the use of subdermal structures such as bloodvessels as the individual's characteristic trait is made possible withthe use of the present invention.

Another aspect of the present invention involves the relationshipexisting between the general wave theory governing the diffractionpattern of ultrasound in the far field and the ability to detect uniquetwo dimensional features. It is known that the far field diffractionpattern of an acoustic wave reflecting off the surface of an object isthe two dimensional Fourier transform of the surface topology of thatobject. Therefore, in accordance with this aspect of the presentinvention, if the ability to perform fingerprint pattern recognition inthe spatial frequency domain can be demonstrated, then an ultrasonicsensor can be provided according to the present invention, to performthis processing automatically as part of the wave propagation. Thefollowing description deals with the concept of fingerprint analysisperformed in the spatial frequency domain.

The problem of two-dimensional pattern recognition and the developmentof various robust algorithms has received much attention over the years.One of the many approaches to automating the recognition oftwo-dimensional patterns has been to analyze, or filter out from theoverall scene, those spatial frequency components not associated with aparticular pattern. That is, considering a two-dimensional pattern as afunction g(x,y), then the spatial frequency components of this functionare given by its two-dimensional Fourier transform:

    F(g(x,y))=G(f.sub.x, f.sub.y)=g(x,y)exp -j2 (f.sub.x x+f.sub.y y)!dxdy

where f_(x) and f_(y) are referred to as the spatial frequencies of gand have units of cycles per unit distance as measured in the planeg(x,y). Thus, by the application of the two-dimensionsal Fouriertransform, the spatial frequency components for a given scene (orpattern) can be obtained. Once these frequencies components have beenobtained, then a spatial filter can be applied to reject or pass thosespatial frequencies associated with the particular pattern that istrying to be identified. A modified scene is then produced by theapplication of an inverse Fourier transform as defined by

    F.sup.-1 (G(f.sub.x, f.sub.y)=G(f.sub.x, y)exp  j2(f.sub.x x+f.sub.y y)!df.sub.x df.sub.y

The modified scene will have only those patterns in it whose spatialfrequency components were allowed to pass. A block diagram of a basicsystem based on the foregoing is given in FIG. 24.

It is known shown how to carry out the Fourier transform opticallythrough the use of a relatively simplistic lensing system. The basis forthis optical transformation is that under certain conditions (andapproximations), the diffraction pattern of a two-dimensional object isitself the two-dimensional Fourier transform of that object. Theseapproximations, one of which is the Fraunhoffer approximation, are oftenmade when dealing with wave propagation. A typical lensing system andits ability to perform spatial filtering is shown in FIG. 25.

As a result of the ability to generate the two-dimensional Fouriertransform optically, a number of systems targeted at characterrecognition have been provided. A common goal was to be able to performpattern recognition on typed characters using matched spatial filters.The overall advantage resided in the fact that the Fourier transform isa linear-shift-invariant operation as well as being unique. That is,there is no loss of information when working with an image in thefrequency domain as compared to the original image data domain. Thisform of coherent processing allows for the parallel searching of allpossible positions of the pattern as opposed to the time consumingprocess of performing template matching on all spatial locations. All ofthese systems experienced varying degrees of success and suffered fromsimilar problems or deficiencies. Specifically, the spatial filter thatwas created was matched (contained both amplitude and phase information)to a particular pattern with a particular orientation and size. It wasnot only difficult to record the phase information for a particularfilter on photographic film, but changes in either orientation or sizecaused an overall degradation in the system's ability to performrecognition. As a result, a number of different approaches were employedto generate both scale and rotationally invariant transformations aswell as implement multiple spatial filters and replace the photographicfilm with a spatial light modulator. Again, all of these approachesexperienced varying degrees of success.

In addition to character recognition, much attention was being directedtowards general scene analysis or pattern recognition in the spatialfrequency domain, and one pattern of particular interest was thefingerprint. One investigator experimented with the concept of lookingat the entire fingerprint in the spatial frequency domain. If theprocess of fingerprint identification in the spatial frequency domaincould be accomplished, then a high speed optical processor could bebuilt to perform the recognition. Essentially, there were threeobstacles to this approach. The first is that the spectrum resultingfrom the 2-D Fourier Transform is so complex and contains so manydifferent, yet significant, spatial frequencies that the matchingprocess becomes virtually impossible. Small variations in scale androtation caused enough of a change in the spectrum so that the matchingprocess became uncertain.

The second problem with this approach lies in the area of compatibility.To date no existing law enforcement agency or known private sectorpersonal identification system uses the approach of viewing the entirefingerprint pattern for identification. In most law enforcement systemsthe entire fingerprint is not stored internal to the computer due to thetremendous storage requirements. Thus, an approach not compatible withthe existing database which has been gathered over several decades wouldhave tremendous difficulty in being adopted or even reviewed.

Finally, since much of the fingerprint identification process isperformed on latent prints where only a partial print is possible,matching the spectrum of the partial print to the spectrum of the entireprint makes partial print identification dificult or impossible.

Despite the above mentioned deficiencies, the need for a new approach tofingerprint processing is constantly growing. The ever increasing numberof fingerprint cards that are processed every day by law enforcementagencies is resulting in large amounts of dedicated custom imageprocessors in order to keep up with demand. In accordance with thepresent invention, by providing an upfront sensor in which much of thecomputationally intensive tasks could be performed as part of thenatural wave propagation at throughputs far exceeding that of thedigital computer, then overall system throughput can be increased whiledecreasing the complexity of the central processing unit. Recently, ithas shown that it is possible to obtain the spatial frequency componentsof the surface topology of an object ultrasonically through pulse-echoimaging. In accordance with the present invention, it is believed thatanalyzing the fingerprint in the spatial frequency domain will beadvantageous. This can be accomplished, according to the presentinvention, by an ultrasonic sensor which is capable of scanning a fingerdirectly and returning its spatial frequency components.

As previously mentioned, automatic fingerprint reader systems (AFRS's)have been in use for a number of years. These devices comprise a numberof highly customized, highspeed, dedicated processors, and their solepurpose is to optically scan an inked image of a fingerprint into acomputer for analysis. The analysis consists of the repeated applicationof a number of image enhancement/pattern recognition algorithms in orderto characterize the fingerprint. The characterization of the fingerprintconsists of identifying the location of each minutiae of the fingerprintin x,y space (an arbitrary but fixed cartesian coordinate system) andits angle of orientation (theta) with respect to one of the axes of thecoordinate system. A minutia is defined as either a bifurcation or ridgeending. That is, when a ridge of a fingerprint that is traveling along aparticular path suddently splits into two separate and distint ridges orcombines with another ridge to form a single ridge, then the transitionpoint is referred to as a bifurcation. Similarly when a ridge that istraveling along a particular path suddenly ends, a ridge ending isformed. It is the identification in x,y,theta of the bifurcations andridge endings, i.e. minutiae, that is used (among other things) todetermine if a match between two prints exist. An example of abifurcation (left) and ridge ending (right) is given in FIG. 26.

An interesting and important observation can be made in studying thebifurcation and ridge ending. That is, a bifurcating ridge is the sameas a ridge ending valley (the space between the ridges). Likewise, aridge ending is the same as a bifurcating valley. In other words, bytaking the complement of an image of a fingerprint (i.e. reverse video),all ridge endings become bifurcations and all bifurcations become ridgeendings. Furthermore, since it is virtually impossible to determinewhether one is viewing a true image of a fingerprint or a reverse videoimage, the decision was made many years ago by the law enforcementcommunity to map both bifurcations and ridge endings. This decision thuseliminated the need to know what type of image was being processed sincethe end result was the same. This is an important fact that forms thebasis of the algorithm employed in the present invention.

Generally, the problems experienced in character recognition systemsbased on matched spatial filters were amplified due to the large numberof different filters required for all the potential patterns andorientations. Given a limited number of patterns with a limited numberof scale and rotational variations, then the above approach becomesworkable.

As previously mentioned, attempts have been made to transform the entirefingerprint into the spatial frequency domain and then perform thematching process. One of the advantages of transforming the image to thespatial frequency domain is the shift-invariance nature of the Fouriertransform requiring only scale and rotational variations to beconsidered. However, these variations were significant obstacles whendealing with a complex spectrum such as that resulting from thetransform of an entire fingerprint. Thus, this approach suffers from thesame problems as does the character recognition schemes. Too manydifferent characters requiring too many different filters or too complexof a frequency spectrum to be easily analyzed. A `typical` fingerprintand its 2-D Fourier transform representation are given in FIG. 27 and28, respectively. It should be noted that FIG. 27 was obtained byoptically scanning an inked image of a fingerprint as opposed to theactual finger itself.

The approach according to the present invention divides the entirefingerprint into n×n windows of m×m size. The window is selected largeenough so as to encompass an entire minutiae (ridge ending orbifurcation) yet small enough to keep the spatial frequency planerepresentation simple. The window is then stepped around the entirefingerprint. After each step a 2-D Fourier transform is performed. Thisis the algorithm according to the present invention. In selecting asmall window, it is easy to identify the presence or absence of aminutia due to the fact that the pattern formed by the spatial frequencycomponents is not only different from a non-minutia window but is anextremely simple pattern to search for. The advantage of frequency planeanalysis is in the shift invariance nature of the Fourier transform. Ifthe window were large enough to encompass the entire minutia, then theminutia could be located in a number of (x,y) positions within thatspace requiring an extensive search in the original plane. Applying theFourier transform to the window prior to analysis reduces thismultipoint search to a single point search. Scale and rotationalvariations still have the same effect on the spatial frequency domainimage but since the pattern being transformed is much simpler by nature,the effects of scale and rotational variations are much more easilyidentifiable than when trying to view the entire print.

FIGS. 29, 30 and 31 are three windows containing at the left-hand sideof each view a bifurcation, ridge ending and no minutia, respectively.The magnitude of the corresponding spatial frequencies resulting from a256×256 point Fast Fourier transform (along with some simple contrastenhancement operators) are also given at the right-hand side of eachview. As is readily seen, the spatial frequency components associatedwith a minutia are quite distinctive and relatively simple.Understanding the spectrums generated by each of the images isimportant. In studying the input image of each minutia, it is easilyseen that the ridge count (the number of fingerprint ridges) when viewedalong the vertical axis is different from one side of the minutia toanother. In the case of the bifurcation shown in FIG. 29, traversing theleft most region of the image along the vertical axis, we obtain a ridgecount of 3, whereas traversing the right most vertical region we obtaina ridge count of 4. Performing a similar process on the image of theridge ending minutia in FIG. 30, we obtain a ridge count of 3 and 2respectively. It is these ridge counts that essentially are transformedinto a particular spatial frequency. Since for a minutia we see that theridge count varies from `side to side`, we obtain 2 spatial frequencycomponents as can be seen by the double dots just above and below theorigin in the frequency spectrum representation found in FIGS. 29 and30. This is in contrast to the non-minutia image of FIG. 31. We see herethat the ridge count of the non-minutia structure does not vary as wetraverse the image but remains a constant value of two. Therefore thetransform results in only a single significant spatial frequency asdepicted by the presence of a single dot just above and below the originof the frequency spectrum representation as found in FIG. 31. The searchfor the presence or absence of a minutia is now reduced to thedetermination of a double versus single spatial frequency, respective(i.e. 2 `dots` verus 1 `dot`). Any small changes in the size and shapeof the minutia merely create a shift in the relative position andspacing of the frequency components. Since we are not interested inabsolute spatial frequency values but are merely looking for a patternformed by the spatial frequencies, these changes in size and shape donot affect the approach. Thus, by definition, the approach is madeinvariant to the size and shape of the minutiae within certain limits.

Searching for the presence of a pattern like with limited scalevariations and unlimited rotational variations is significantly easierthan the spectrum presented earlier in FIG. 28. It should be noted thatthe only difference between the spectrums of the bifurcation versus theridge endings lies in those frequency components associated with theridge structure directly at and immediately surrounding the junction ofthe bifurcation. These components are so small however that they areeasily removed through thresholding, resulting in a spatial frequencypattern virtually identical for either ridge endings or bifurcations.

In order to establish some basis for acceptance, a portion of afingerprint given in FIG. 27 was processed using spatial frequencyanalysis. The results of this processing was then visually checkedagainst the actual image to determine if any minutiae failed to beidentified or, conversely, any false minutia were reported. The resultsof this processing resulted in the detection of all six minutia presentin the print of FIG. 27. The output of the 2-D Fourier transform foreach of the detected minutia is presented in FIGS. 32a-f, respectively.Furthermore, FIG. 33 provides a map of the detected minutiae indicatedby a circle and their direction as indicated by the "tail" of thecircle.

By way of illustration, the implementation of such a technique usingultrasonic diffraction would require the replacement of the singleelement transducer 140 with a multielement phased array transucer andfor each element of the phased array transucer, a copy of the signalprocessing electronics as presented herein in FIG. 21. The cost of sucha system would be significant unless the multiple copies of the signalprocessing electronics could be reduced using custom integratedcircuitry.

It is therefore apparent that the present invention accomplishes itsintended objects. Ultrasound has been employed as an alternate approachto optical surface scanning based on frustrated total internalreflection. This new approach has the potential for better performancethan previously used optical approaches by eliminating many of theproblems associated with the optical approach. Feasibility has beendemonstrated through consideration of available acoustic imaging systemsas well as providing an actual system. Fingerprint images of highquality have been captured to demonstrate that concerns such as lateralresolution and repeatability have been overcome. The approach of thepresent invention is quite feasible and even preferable over the opticalapproach based on Frustrated Total Internal Reflection. The presentinvention also employs ultrasound as a means of obtaining an image ofthe ridge structure of the finger. Furthermore, the data collected andpresented clearly shows an image of high quality, absent of themultitude of discontinuities characteristic of the optical based systemsdue to the inability to image through small air pockets that becometrapped between the finger and the surface of the scanner. The approachof analyzing the fingerprint image in the spatial frequency domain alsois presented.

What is claimed is:
 1. A fingerprint imaging method comprising the stepsof:a. placing a live finger upon a scannable surface of a body ofmaterial having an acoustic impedance substantially matching that of theskin of the finger for imaging the same over the area of an image plane;b. scanning the portion of the finger on said surface by focusing anddirecting an ultrasonic energy beam onto said surface and in a directionalways substantially perpendicular to said image plane so as to providea minimum spot size at the focal point of said beam to provide maximumlateral resolution imaging in a plane substantially perpendicular to thedirection of said beam; c. receiving ultrasonic energy returned fromsaid finger portion to capture an electronic image of the pattern ofridges and valleys of the fingerprint; and d. said step of scanningincluding propagating the ultrasonic energy through a liquid mediumproviding relatively low attenuation at the frequency of the ultrasonicenergy.
 2. A method according to claim 1, wherein said step of sacanningis performed with ultrasonic energy at a frequency of a at least 15 MHZ.3. A method according to claim 1, wherein said step of scanning isperformed with ultrasonic energy at a frequency of about 30 MHZ.
 4. Amethod according to claim 1, wherein said step of scanning is performedby sweeping the spot across the portion of the finger on the surface insector-life fashion.
 5. A method according to claim 4, further includingcollimating said beam.
 6. Apparatus for fingerprint imagingcomprising:a. a body having a scannable surface upon which a live fingeris placed, said body being of material having an acoustic impedancesubstantially matching that of the skin of the finger for imaging thesame over the area of an image plane; b. means for providing anultrasonic beam focused and directed onto said surface and in adirection always substantially perpendicular to said image plane so asto have a minimum spot size at the focal point of said beam to providemaximum lateral resolution imaging in a plane substantiallyperpendicular to the direction of said beam; c. means for scanning theportion of the finger on said surface using said ultrasonic beampropagated through a liquid medium providing relatively low attenuationat the frequency of the ultrasonic beam and in a manner providing highlateral resolution imaging; and d. means for receiving ultrasonic energyreturned from said finger portion to capture an electronic image of thepattern of ridges and valleys of the fingerprint.
 7. A method accordingto claim 1, wherein said ultrasonic beam is provided by a transducerwhich is shaped at the output end thereof in a manner providing focusingof the beam.
 8. Apparatus according to claim 6, wherein said means forproviding an ultrasonic beam comprises a transducer which is shaped atthe output end thereof in a manner providing focusing of the beam.